Malaria

Overview & Epidemiology

Malaria is a potentially fatal parasitic disease transmitted by the bite of the female Anopheles mosquito. Over 41% of the world's population lives in endemic areas (parts of Africa, Asia, Oceania, Central and South America). The WHO estimated 219 million cases and ~435,000 deaths annually, primarily in children infected with P. falciparum in sub-Saharan Africa. Between 2000 and 2022, the WHO African Region reported a 40% reduction in incidence and a 60% decline in mortality, though cases have been rising again in recent years.

Murray & Nadel's Textbook of Respiratory Medicine, p. 1352

Causative Agents (Species)

Five Plasmodium species infect humans:

Species	Periodicity of fever	Notes
P. falciparum	Every 48 hrs	Most dangerous; nearly universally chloroquine-resistant
P. vivax	Every 48 hrs	Has dormant liver forms (hypnozoites)
P. ovale	Every 48 hrs	Has dormant liver forms
P. malariae	Every 72 hrs	Lower severity
P. knowlesi	Every 24 hrs	Zoonotic (simian); virulent, can be fatal; morphologically resembles P. malariae

Life Cycle & Pathogenesis

Infected female Anopheles mosquito injects sporozoites into the bloodstream
Sporozoites travel to the liver (clinically silent hepatic phase, ~6-7 days)
Thousands of merozoites are released and infect red blood cells (RBCs)
The erythrocytic stage causes clinical disease - lysis of infected RBCs triggers fever

In P. falciparum, infected erythrocytes express variant antigens that cause them to adhere to microvasculature (sequestration), which is central to cerebral malaria pathogenesis. Inflammatory cytokines and vascular adhesion molecules also play key roles.

Genetic erythrocyte disorders (sickle cell trait, thalassemia, G6PD deficiency) are most prevalent in malaria-endemic regions - heterozygous HbS (sickle cell trait) is protective against severe malaria.

Clinical Features

Classic triad: Chills - fever - sweating (periodic)
Headache, myalgia, nausea, vomiting, diarrhea
Fever may be continuous early on, becoming periodic later
Fever is common but not universal at initial presentation

Severe malaria (P. falciparum primarily):

Cerebral malaria (coma, seizures)
Severe anemia
Thrombocytopenia
Jaundice
Acute kidney injury (renal failure)
Acute respiratory distress syndrome (ARDS) / non-cardiogenic pulmonary edema
Hypoglycemia
Multi-organ failure (can progress within days in non-immune patients)

Pulmonary involvement: Bilateral opacities on chest X-ray resembling pulmonary edema; alveolar macrophages may contain hemozoin (a brown hemoglobin degradation product from the parasite).

Diagnosis

Peripheral blood smear - gold standard; shows parasitized RBCs. May be negative initially - repeat after 8 hours if clinical suspicion is high
Rapid diagnostic tests (RDTs) - detect P. falciparum and P. vivax antigens; point-of-care; widely used in endemic settings (note: antigen can persist, so doesn't reliably distinguish current from very recent infection)
PCR - not widely commercially available; highly sensitive

A parasitemia >5% (percentage of parasitized RBCs) signals severe disease.

Treatment

Treatment depends on species and geographic origin (resistance patterns):

Situation	Treatment
P. vivax, P. ovale, P. malariae, P. knowlesi	Chloroquine (still effective)
Uncomplicated P. falciparum	Artemisinin-based combination therapies (ACTs) - first line
Severe malaria (any species with organ involvement)	IV artesunate - treatment of choice
P. vivax / P. ovale (relapse prevention)	Primaquine (to eliminate hypnozoites) - check G6PD status first

Note: IV quinine is no longer available in the US (since March 2019). IV artesunate is the only option for severe malaria in the US; providers can contact the CDC Malaria Hotline: 855-856-4713.

Artesunate resistance has been increasingly detected in P. falciparum, especially in Southeast Asia, manifesting as slower parasitemia clearance.

Supportive care is essential: careful fluid balance, correction of hypoglycemia, oxygen support. Concurrent bacterial sepsis or pneumonia must be treated with appropriate antibiotics.

Prevention

Chemoprophylaxis for travelers to endemic regions (options: mefloquine, atovaquone-proguanil, doxycycline)
Insecticide-treated bed nets (ITNs)
Indoor residual spraying (IRS)
Vaccines: RTS,S/AS01 (Mosquirix) and R21/Matrix-M are now deployed for children under 5 in endemic areas
Seasonal malaria chemoprevention (SMC) and intermittent preventive treatment in pregnancy (IPTp)

According to the WHO 2025 malaria guidelines, ACTs remain the most effective and widely recommended treatment, and global funding gaps (US$3.9B available vs. US$9.3B needed) continue to challenge control efforts.

Key Points to Remember

P. falciparum is the most lethal species and is chloroquine-resistant nearly everywhere
Most deaths in developed countries result from delayed diagnosis
Always obtain a thorough travel history in any febrile returning traveler
Thick and thin blood smears should be repeated if initially negative
Primaquine is required to prevent relapses in P. vivax and P. ovale - always check G6PD status first
ROSEN's Emergency Medicine, p. 1218 | Murray & Nadel's Respiratory Medicine, pp. 1351-1353

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